The present invention relates to an efficient method for radiation sensitive film dosimetry, and in particular, to methods for measuring a distribution of ionizing radiation doses.
Radiotherapy has been used for years as a method for irradiating and selectively killing cancer cells while minimizing radiation exposure to adjacent tissue. The effectiveness of radiotherapy depends upon the absorbed dose or the amount of energy deposited within a tissue mass. Absorbed dose is typically measured in centigray or cGy units.
A radiation detection medium may be used to determine the amount and location of radiation to which a patient is subjected during radiation treatment. Particularly useful is a two-dimensional radiation detection medium that can determine radiation dose over an area. Examples are radiographic film, radiochromic film, phosphor plates, two-dimensional arrays of diodes or ion chambers and the like. The radiation detection medium typically has a response that varies systematically in accordance with the degree of radiation exposure. After exposure to ionizing radiation, radiation detection media such as radiographic and radiochromic films typically have a light transmission or optical density that varies systematically in proportion to the radiation dose. Calibration of the radiation sensitive film allows one to measure the absorbed dose indirectly by measuring the light transmission or optical density of the exposed radiation detection medium.
Radiochromic films are widely used by Medical Physicists to perform radiation dosimetry. Film is widely recognized as a “gold standard” for performing radiation dosimetry because of its exceptional spatial resolution, reaching to a level of at least 0.025 mm. Such high resolution is not possible with other measurement modalities including ion chambers, diodes and TLD. These other techniques are limited to spatial resolution in the range of 1 mm or coarser. Another advantage of radiochromic film is its tissue equivalence that the adsorbed radiation dose is truly reflection of the dose adsorbed by tissue.
Calibration curves for a radiation detection medium are often prepared by exposing one or more areas of the detection medium to different and known amounts of radiation using a linear accelerator or a similar device capable of generating a range of known dose levels. Another method frequently used is to expose the detection means to a continuously varying level of doses. This can be done by interposing a wedge of material with continuously varying thickness between the radiation source and the detection medium. Alternatively, the radiation sensitive medium may be sandwiched between two blocks and positioned so that the medium is in a plane parallel to the beam when it is exposed. In this configuration the dose applied to the radiation sensitive medium decreases continuously with depth below the top surface of the blocks. This type of exposure is often referred to as a depth-dose exposure. Typically, calibration curves are generated by measuring the response of the radiation sensitive medium for numerous different dose levels. In the instance of radiation films, it is common to measure the light transmission or optical density of the medium for numerous different radiation dosage levels.
Until now, a disadvantage of dosimetry with radiochromic film relative to the use arrays of ion chambers and diodes, is that it is less time-efficient. For example, commercial devices comprising arrays of ion chambers or diodes are commonly used to validate intensity modulated radiotherapy treatment (IMRT) plans delivered by linear accelerators. The time needed to set-up the array device on a linear accelerator, make the measurement, compare the measured values with the treatment plan and then pack up the device is of the order of 30 to 40 minutes. On top of this is an overhead for calibration of the array. However, this overhead is small since the array may be calibrated only once every few months during which time dozens of validation measurements would be made.
For a number of reasons it has required a significantly longer time to do a similar measurement using radiochromic film. Also, the use of film has brought other inconveniences. Firstly, the calibration of radiochromic film can be inefficient in that new production lots of film require new calibration. —The time required to set up and expose film to an IMRT plan is less than is required for the array devices. However, once the film has been exposed it must be scanned on a film digitizer and then the scan data must be processed to produce a measurement result. Usually a measurement with film will have required multiple scans and on top of that the scanning must be done at a well regulated time-after-exposure corresponding to the time-after-exposure of the calibration film. The restriction of time-after-exposure is due to the fact that the polymerization reaction initiated by the exposure continues after the exposure at a rate that diminishes with time. In practical terms this meant that several hours elapsed between exposure and scanning, meaning that the results of the measurement are only available after a considerable and inconvenient time delay. In total, the time required for the film measurement can be as much as 3× the time required to make measurements with the array devices.
As alluded to above, though the spatial resolution of a single ion chamber or diode is on the order of 1 mm, the array devices can comprise many hundreds or even a few thousand devices, but in order to cover a large area, typically 400-1000 cm2, the individual sensors can be 5-10 mm apart, severely decreasing the spatial resolution of the array device. Since these devices are used to validate treatment plans calculated at a resolution of 1 mm, or better, the array devices are not sufficient to their function.
Having spatial resolution orders of magnitude better than 1 mm, radiochromic dosimetry film could be the best choice for dose measurement and radiotherapy treatment plan validation if it were more convenient and faster to use.